Nano/micro-sized diode and method of preparing the same

ABSTRACT

A nano/micro-sized diode and a method of preparing the same, the diode including: a first electrode; a second electrode; and a diode layer that is disposed between the first electrode and the second electrode. The diode layer includes a first layer and a second layer. The first layer is disposed on the first electrode and has a first surface that is electrically connected to the first electrode, and an opposing second surface that has a protrusion. The second layer is disposed between the first layer and the second electrode and has a first surface having a recess that corresponds to the protrusion, and an opposing second surface that is electrically connected to the second electrode.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This is a divisional of U.S. application Ser. No. 12/240,156, filed Sep.29, 2008, which claims benefit of Korean Patent Application No.:10-2008-16371, filed Feb. 22, 2008, in the Korean Intellectual PropertyOffice, the disclosures of which are incorporated herein, by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a nano/micro-sized diode anda method of preparing the same.

2. Description of the Related Art

Currently, there is a trend toward producing electronic devices that aresmaller and have faster operational speeds/capabilities. However, due tothe limitations of conventional technologies, there is a need to developnew technologies to address these challenges. Thus, much research isbeing conducted into nanoscience and nanotechnology. Nanotechnology isused to make nanometer scale materials that can maximize informationstorage and processing. Thus, nanotechnology is drawing much attentionas a technology for use in industries, such as information technology,biotechnology, etc. Nanoscience is divided into two fields: thesynthesis of materials, such as carbon nanotubes, C60, mesoporousmaterials, metallic and semiconductor nanocrystallines (nanocrystal,nanocluster, quantum dot), or the like; and the control and applicationof nanomaterials, using scanning tunneling microscopy (STM), atomicforce microscope (AFM), or lithography.

Nano electro mechanical system (NEMS) technology and micro electromechanical system (MEMS) technology refer to technologies related tominiature precision machinery. These technologies are expected tooutperform current semiconductor technology. The NEMS and MEMStechnologies are derived from semiconductor technology, relate tothree-dimensional space, and are being researched for applyingnanotechnology in various types of devices. Thus, research intonanoparticles, nanowires, nano-multilayer structures, and the like isbeing actively conducted. Methods of electrochemically preparing suchnanostructures are drawing much attention because of various advantages,such as cost reduction, design simplicity, and flexibility in thebuilding complex shapes.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anano/micro-sized diode comprising: a first electrode; a secondelectrode; and a diode layer that is disposed between the firstelectrode and the second electrode. The diode layer comprises a firstlayer and a second layer. The first layer is disposed between the firstelectrode and the second layer, and the second layer is disposed betweenthe first layer and the second electrode. The first layer has a firstsurface that is electrically connected to the first electrode, and anopposing second surface that has a protrusion. The second layercomprises a first surface having a recess that corresponds to theprotrusion of the first layer, and an opposing second surface. Thesecond surface of the second layer is electrically connected to thesecond electrode.

According to aspects of the present invention, the nano/micro-sizeddiode may further comprise a control layer disposed between the secondelectrode and the diode layer. The second surface of the second layermay be electrically connected to the control layer.

According to aspects of the present invention, the first surface of thesecond layer may entirely cover the second surface of the first layer.

According to aspects of the present invention, the second layer may bewider than the first layer.

According to another aspect of the present invention, there is provideda method of preparing a nano/micro-sized diode, the method comprising:preparing a porous template comprising a plurality of holes; formingfirst electrodes in the bottom of the holes; forming a material layer onthe first electrodes; heat-treating the material layer to form firstlayers, which have first surfaces that are electrically connected to thefirst electrodes, and opposing second surfaces having protrusions;forming second layers on the first layers, the second layers havingfirst surfaces having recesses corresponding to the protrusions; formingsecond electrodes on the second layers; and removing the poroustemplate, to obtain a plurality of nano/micro-sized diodes.

According to aspects of the present invention, the method of preparing anano/micro-sized diode may further comprise forming control layers onthe second layers.

According to aspects of the present invention, the method of preparing anano/micro-sized diode may further comprise expanding a space betweenthe protrusions and the walls of the holes.

According to aspects of the present invention, the nano/micro-sizeddiode has excellent durability. In addition, by using the preparationmethod of the present invention, nano/micro-sized diodes can bemass-produced at a high yield, with a uniform size and excellentquality.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1A is a schematic diagram illustrating a nano/micro-sized diode,according to an exemplary embodiment of the present invention;

FIG. 1B is a schematic cross-sectional view taken along line I-I′ ofFIG. 1A;

FIG. 2A is a schematic diagram illustrating a nano/micro-sized diode,according to another exemplary embodiment of the present invention;

FIG. 2B is a schematic cross-sectional view taken along line I-I′ ofFIG. 2A;

FIG. 3A is a schematic diagram illustrating a nano/micro-sized diode,according to another exemplary embodiment of the present invention;

FIG. 3B is a schematic cross-sectional view taken along line I-I′ ofFIG. 3A;

FIGS. 4A through 4G are cross-sectional views sequentially illustratinga method of preparing a nano/micro-sized diode, according to anexemplary embodiment of the present invention;

FIGS. 5A through 5H are cross-sectional views sequentially illustratinga method of preparing a nano/micro-sized diode, according to anotherexemplary embodiment of the present invention;

FIGS. 6A through 6D are field emission scanning electron microscope(FESEM) images of the production process of a nano/micro-sized diode,according to an exemplary embodiment of the present invention;

FIGS. 7A and 7B are FESEM images, taken with different magnifications,of a nano/micro-sized diode, according to an exemplary embodiment of thepresent invention;

FIG. 8A is a scanning electron microscope (SEM) image of anano/micro-sized diode, according to an exemplary embodiment of thepresent invention;

FIGS. 8B through 8D are energy dispersive spectroscopy (EDS) spectra ofa nano/micro-sized diode, according to an exemplary embodiment of thepresent invention;

FIG. 9 is a graph showing voltage-current characteristics of anano/micro-sized diode, according to an exemplary embodiment of thepresent invention; and

FIG. 10 is a graph showing voltage-current characteristics of anano/micro-sized diode, according to another exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Reference will now be made in detail to the exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The exemplary embodiments are described below, in order toexplain the aspects of the present invention, by referring to thefigures.

FIG. 1A is a schematic diagram illustrating a nano/micro-sized diode 10,according to an exemplary embodiment of the present invention. FIG. 1Bis a schematic cross-sectional view taken along line I-I′ of FIG. 1A.

The term “nano/micro sized” refers to a parameter that can define thesize of a diode, for example, at least one of length, diameter, andwidth, in units ranging from several nanometers to several tens ofmicrometers. As referred to herein, when a first element is said to bedisposed or formed “on”, or “adjacent to”, a second element, the firstelement can directly contact the second element, or can be separatedfrom the second element by one or more other elements locatedtherebetween. In contrast, when an element is referred to as beingdisposed or formed “directly on” another element, there are nointervening elements present. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

Referring to FIGS. 1A and 1B, the nano- or micro-sized diode 10 includesa first electrode 11, a diode layer 15, and a second electrode 13, whichare sequentially stacked. The first electrode 11 may comprise aconductive material, and in particular, an electrochemically stableconductive material. The first electrode 11 may comprise a materialselected from the group consisting of Pt, Au, Al, Ni, Mo, W, ITO,carbon, carbon nanotubes, and conducting polymers; however, the presentinvention is not limited thereto.

In the diode layer 15, a net current flows, and a reverse current isblocked. The diode layer 15 includes a first layer 15 a and a secondlayer 15 b. The first layer 15 a is disposed between the first electrode11 and the second layer 15 b. The second layer 15 b is disposed betweenthe first layer 15 a and the second electrode 13.

The first layer 15 a has a first surface 15 a ₁ that is electricallyconnected to the first electrode 11, and an opposing second surface 15 a₂ that has a protrusion. In FIG. 1B, the protrusion has a rectangularcross-section. However, this is for schematic illustration purposesonly, and the protrusion can be any suitable shape.

The second layer 15 b has a first surface 15 b ₁, having a recesscorresponding to the protrusion, and an opposing second surface 15 b ₂that is electrically connected to the second electrode 13.

The first layer 15 a may comprise a material having p-type semiconductorproperties. Examples of the material include conducting polymers thathave polymeric mechanical properties that can be transferred to asemiconductor or conductor, from an insulator, through chemical doping.Examples of the conducting polymers include polypyrrole, polyaniline,polythiophene, polypyridine, polyazulene, polyindole, polycarbazole,polyazine, polyquinone, poly(3,4-ethylenedioxythiophene), polyacetylene,polyphenylene sulfide, polyphenylene vinylene, polyphenylene,polyisothianaphthene,poly(2-methoxy-5-(2′-ethyl)hexyloxy-p-phenylenevinylene (MEH-PPV), amixture of polyethylenedioxythiophene (PEDOT) and polystyrenesulfonate(PSS), polyfuran, polythienylene vinylene, and a derivative thereof.However, the present invention is not limited thereto. Alternatively,the first layer 15 a may comprise a metal oxide having p-typesemiconductor properties. Examples of the metal oxide include indium-tinoxide (ITO), indium-zinc oxide (IZO), and the like. However, the presentinvention is not limited thereto.

The second layer 15 b may comprise a semiconducting material. Examplesof the semiconducting material include cadmium selenide (CdSe), cadmiumtelluride (CdTe), cadmium sulfide (CdS), zinc oxide (ZnO), and the like.However, the present invention is not limited thereto. The CdSe refersto an alloy of cadmium (Cd) and selenium (Se) that has n-typesemiconductor properties and can act as a photocell when irradiated withlight.

As described above, the diode layer 15 may include the p-type firstlayer 15 a and the n-type second layer 15 b, so that a p-n junction isformed therebetween. Thus, when light enters the diode layer 15, or avoltage is applied thereto, electrons are transferred to the secondelectrode 13, via the second layer 15 b, and holes are transferred tothe first electrode 11, via the first layer 15 a. As a result,electrical signals can be generated. Therefore, a diode, in whichelectrons and holes can be readily separated, can be provided.

The diode layer 15 includes the first layer 15 a and the second layer 15b. The second surface 15 a ₂ of the first layer 15 a has the protrusion,and the first surface 15 b ₁ of the second layer 15 b has the recessthat corresponds to the protrusion. The second surface 15 a ₂ iscompletely covered by the first surface 15 b ₁. Thus, the interfacebetween the second surface 15 a ₂ and the first surface 15 b ₁ can berelatively increased. Due to this, the diode layer 15 is mechanicallyvery strong. Therefore, during preparation of the nano/micro sized diode10, a yield reduction, caused by a break down of the diode layer 15, canbe prevented. In addition, during storage and transportation of thediode 10, damage caused by the break down of the diode layer 15 can beprevented.

The second electrode 13 is formed on the diode layer 15. The secondelectrode 13 is electrically connected to the second surface 15 b ₂ ofthe second layer 15 b. A material used to form the second electrode 13may be conductive, and in particular, may be an electrochemically stableconductive material. Examples of the material used to form the secondelectrode 13 include Pt, Au, Al, Ni, Mo, W, ITO, carbon, carbonnanotubes, and conducting polymers. However, the present invention isnot limited thereto.

The nano/micro sized diode 10 illustrated in FIG. 1A is in the form of acylindrical nanorod. The nano/micro sized diode 10 has a length a(thickness) and/or a cross-section width b (diameter) in the range ofseveral nanometers to tens of micrometers. For example, the length a ofthe nano/micro sized diode 10 may be in the range of from 500 nm to 100μm, and the diameter b may be in the range of 30 to 1000 nm. However,the present invention is not limited thereto, and the length anddiameter of the nano/micro sized diode 10 may vary, according to thereaction conditions when the diode 10 is prepared. In addition, a lengthratio, of each part of the nano/micro sized diode 10 illustrated in FIG.18, may vary according to reaction conditions. For example, a ratio ofa₁:a₂:a₃ of the nano/micro sized diode 10 may be 30:15:20, and a ratioof b₁:b₂:b₃ may be 1:15:1. However, the present invention is not limitedthereto.

Although not specifically illustrated in the drawings, the nano/microsized diode 10 may have different shapes and/or sizes, according to theshape and/or size of a hole of a porous template used in the preparationprocess. In addition to the nanorod form illustrated in FIGS. 1A and 18,the nano/micro sized diode 10 may be in the form of a nanowire, ananoneedle, a nanobelt, a nanoribbon, or the like; however, the presentinvention is not limited thereto.

FIG. 2A is a schematic diagram illustrating a nano/micro-sized diode 20,according to another exemplary embodiment of the present invention. FIG.2B is a schematic cross-sectional view taken along line I-I′ of FIG. 2A.The nano/micro-sized diode 20 includes a first electrode 21, a diodelayer 25, a control layer 27, and a second electrode 13, which aresequentially stacked.

The diode layer 25 includes the first layer 25 a and the second layer 25b. The first layer 25 a has a first surface 25 a ₁ that is electricallyconnected to the first electrode 21, and a second surface 25 a ₂ thatfaces the first surface 25 a ₁ and has a protrusion. The second layer 25b has a first surface 25 b ₁ having a recess corresponding to theprotrusion, and an opposing second surface 25 b ₂ that is electricallyconnected to the control layer 27. The first electrode 21, the diodelayer 25, and the second electrode 23 are similar to those of FIGS. 1Aand 1B, and a detailed description thereof is omitted.

The control layer 27 allows holes and electrons separated from the diodelayer 25 to smoothly flow into the first electrode 21 and the secondelectrode 23, respectively. The control layer 27 may comprise a materialselected from the group consisting of Ag, Cu, and Al. However, thepresent invention is not limited thereto. For the transfer of electronsfrom the diode layer 25, the first electrode 21 and the control layer 27may comprise materials that have different work functions.

The nano/micro sized diode 20 illustrated in FIG. 2A is a cylindricalnanorod. The nano/micro sized diode 20 has a length c (thickness) and/ora cross-section width d (diameter) in the range of several nanometers totens of micrometers. For example, the length c of the nano/micro sizeddiode 20 may be in the range of from 500 nm to 100 μm, and the diameterd thereof may be in the range of 30 to 1000 nm. However, the presentinvention is not limited thereto, and the length and diameter of thenano/micro sized diode 20 may vary, according to reaction conditions. Inaddition, a length ratio of each part of the nano/micro sized diode 20may vary according to reaction conditions. For example, a ratio ofc₁:c₂:c₃: c₄ of the nano/micro sized diode 20 may be 30:15:5:20, and aratio of d₁:d₂:d₃ may be 1:15:1. However, the present invention is notlimited thereto.

FIG. 3A is a schematic diagram illustrating a nano/micro-sized diode 30,according to another exemplary embodiment of the present invention. FIG.3B is a schematic cross-sectional view taken along line I-I′ of FIG. 3A.

The nano/micro-sized diode 30, of FIGS. 3A and 3B, includes a firstelectrode 31, a diode layer 35, a control layer 37, and a secondelectrode 33, which are sequentially stacked. The diode layer 35includes the first layer 35 a and the second layer 35 b. The first layer35 a has a first surface 35 a ₁ that is electrically connected to thefirst electrode 31, and an opposing second surface 35 a ₂ that has aprotrusion. The second layer 35 b has a first surface 35 b ₁ having arecess corresponding to the protrusion, and an opposing second surface35 b ₂ that is electrically connected to the control layer 37.

A portion 35 b ₃ of the first surface 35 b ₁ of the second layer 35 bdoes not contact the second surface 35 a ₂ of the first layer 35 a. Sucha structure can be formed by expanding a space between the protrusionand the walls of a hollow channel of a porous template, before formingthe second layer 35 b, and after forming the first layer 35 a. As aresult, the second layer 35 b can be strengthened. The first electrode31, the diode layer 35, the control layer 37, and the second electrode33 are similar to those of FIGS. 1A, 1B, 2A, and 2B, and a detaileddescription thereof, is omitted.

Nano/micro-sized diodes, according to aspects of the present invention,have been described with reference to FIGS. 1A through 3B. However, thepresent invention is not limited thereto, and various other embodimentsare possible. For example, the control layer 37, of the nano ormicro-sized diode 30 of FIG. 3A, can be omitted.

A nano/micro-sized diode, according to aspects of the present invention,can be used in opto-devices, optical sensors, solar cells, energysources for nano electro mechanical systems (NEMS), micro electromechanical systems (MEMS), optical switches, sensors for a chemicalmaterial or biochemical material, and the like. Opto-devices are lightemitting devices that convert an electrical signal to an optical signal,or an optical signal into an electrical signal, and include any kind ofdevice that modulates or mixes light with an electrical signal.

The nano/micro-sized diode, according to aspects of the presentinvention, can be mass produced using a porous template that includes aplurality of holes. Herein, the diode layer of the nano/micro-sizeddiode has the first layer having the protrusion and the second layerhaving the recess corresponding to the protrusion, and thus, thenano/micro-sized diode can have improved mechanical durability.Therefore, the breakage of the diode layer during the preparation of thediode, and the like, can be substantially prevented, and thus, thenano/micro-sized diode can be obtained with high yield.

FIGS. 4A through 4G are views sequentially illustrating a method ofpreparing a nano/micro-sized diode, according to an exemplary embodimentof the present invention, and simultaneously illustrate thecross-section of a porous template and the cross-sections of thenano/micro-sized diodes formed therein. As illustrated in FIG. 4A, aporous template 50, including a plurality of holes 51, is prepared. Theholes 51 may have diameters of from 250 to 300 nm, but the diameters ofthe holes 51 can be altered, according to the types of nano/micro-sizeddiodes to be formed.

The material of the porous template 50 is not particularly limited.Examples of the material of the porous template 50 include an anodicaluminum oxide (AAO) membrane, a polycarbonate template, an anodictitania membrane, a porous membrane of a polymer includingpolypropylene, nylon, polyester, and block copolymers thereof, and thelike. The shapes and sizes of the holes 51 are not particularly limited.

The porous template 50 may be an AAO membrane, which can be prepared byanodization. The holes 51 of the porous template 50 are regularlyarranged, and the arrangement, lengths, and diameters of the holes 51can be adjusted, according to conditions in the preparation of theporous template 50. In the case of the AAO membrane, the lengths anddiameters of the holes 51 can be adjusted according to the oxidationconditions during the anodization of the membrane, i.e., the types ofsolutions, the oxidation temperatures, electric potential differencesbetween the anode and cathode, oxidation times, and the like.

As illustrated in FIG. 4B, first electrodes 41 are formed in one end(the bottoms) of the holes 51. When the first electrodes 41 are formedby electroplating, a working electrode is first formed on the bottomportion of the porous template 50, although this is not illustrated inFIG. 4B. For example, the working electrode may be an Ag electrode, acounter electrode may be a platinum wire mesh, and a reference electrodemay be an Ag/AgCl electrode, a calomel electrode, a standard hydrogenelectrode, or the like. However, the present invention is not limitedthereto. When the first electrodes 41 are formed by electroplating, acommonly used electropolymerization device can be used. Herein, theelectropolymerization device may include a potentiostat that canmaintain a constant voltage. A conventional plating solution, in whichthe material of the first electrodes 41 is dissolved, may be used as aplating solution. For example, when the Au first electrodes 41 areformed by electroplating, the plating solution may be an Orotemp 24 RTUsolution (Technic, Inc.). A voltage used in electroplating may vary,according to the material of the first electrodes 41, but may be in therange of approximately −0.9 to −1 V.

As illustrated in FIG. 4C, material layers 45′ are formed on the firstelectrodes 41. The material layers 45′ may be formed by, for example, anelectrochemical polymerization, in which a mixture including a firstlayer precursor and a solvent is used. That is, an electrochemicalpolymerization device is filled with the mixture, and the poroustemplate 50 is immersed in the mixture. Then, when a current or voltageis applied, the precursor is electrochemically oxidized (polymerizationor extraction) onto the first electrodes 41 in the holes, to form thematerial layers 45′. The material layer 45′ includes the precursor, thesolvent, and a material for forming the first layers. The material layer45′ can include a conductive polymer, and the first layer precursor maybe a monomer of the conducting polymer. In addition, the solvent may bea solvent that can dissolve the monomer (for example,tetraethylammoniumtetrafluoroborate, acetonitrile, or the like). Forexample, when pyrrole is used as the first layer precursor, andtetraethylammoniumtetrafluoroborate is used as the solvent, the materiallayer 45′ includes pyrrole, tetraethylammoniumtetrafluoroborate, andpolypyrrole.

The conditions of the electrochemical polymerization may vary, accordingto the composition of the material layers 45′. For example, when thematerial layer 45′ comprises polypyrrole, the electrochemicalpolymerization may be performed at a voltage of 0.9 V, for apolymerization time of about 10 seconds.

Next, the porous template 50 including the material layers 45′ isheat-treated, to remove the solvent and the precursor. As illustrated inFIG. 4D, first layers 45 a, which each have a first surface 45 a ₁ thatis electrically connected to the first electrode 41, and a secondsurface 45 a ₂ having a protrusion, are formed. That is, the precursoris removed, to contract the material layers 45′, and consequently, formthe protrusions on the second surfaces 45 a ₂. Thus, spaces, between theprotrusions and the walls of the holes 51, may be formed.

Herein, the size and shape of the protrusion can be controlled byadjusting a heat treatment temperature and time, during the formation ofthe first layers 45 a. For example, the material layers 45′ may beheat-treated at a temperature in the range of 25 to 150° C., for 1 to 24hours; however, the present invention is not limited thereto.

Next, a material for forming second layers 45 b is disposed on the firstlayers 45 a. The second layers 45 b each have a first surface 45 b ₁having recesses corresponding to the protrusions of the second surfaces45 a ₂, as illustrated in FIG. 4E. The second layers 45 b can be formedby immersing the porous template 50 in a mixture including the materialfor forming the second layers, and then electrochemically reducing thematerial. The material can surround the protrusions. Thus, the secondlayers 45 b can be formed, as illustrated in FIG. 4E.

By forming the first layers 45 a and the second layers 45 b using themethods described above, a diode layer of the nano/micro-sized diode hasexcellent mechanical durability. Therefore, the diode layers areprotected from damage, during the mass production of the diode. Inaddition, the diodes are protected from damage during storage andtransportation.

Next, as illustrated in FIG. 4F, a control layer 47 is formed on each ofthe second layers 45 b. As illustrated in FIG. 4G, a second electrode 43is formed on each of the control layers 47. The control layers 47 andthe second electrodes 43 may be formed by the electroplating method usedto form the first electrodes 41.

Lastly, the porous template 50 is removed, to obtain a plurality ofnano/micro-sized diodes. The porous template 50 may be removed using aconventional method, such as wet etching, dry etching, photoetching,pyrolysis, or the like.

The wet etching is performed using an etchant, which is an acid or basethat selectively removes only the porous template 50, such as, a sodiumhydroxide solution, an aqueous acetic acid solution, hydrofluoric acid,an aqueous phosphoric acid solution, or the like. The dry etching isperformed using a gas, a plasma, an ion beam, or the like. Examples ofthe dry etching include reactive ion etching (RIE), in which a reactivegas plasma is activated, to chemically volatize with the porous template50, and inductively coupled plasma reactive ion etching (ICP-RIE), inwhich the ICP is used as an activation source.

When the nano/micro-sized diodes 30 are prepared using the poroustemplate 50, the lengths of the first electrode, the diode layer, thecontrol layer, and the second electrode are adjusted, by monitoring thecharges passing through each layer. Accordingly, the nano/micro-sizeddiode 30 can be mass-produced at a high yield.

FIGS. 5A through 5H are cross-sectional views sequentially illustratinga method of preparing a nano/micro-sized diode, according to anotherexemplary embodiment of the present invention. As illustrated in FIG.5A, a porous template 70, including a plurality of holes 71, isprepared.

Next, as illustrated in FIG. 5B, first electrodes 61 are formed in thebottoms of the holes 71. As illustrated in FIG. 5C, material layers 65′are formed on the first electrodes 61 and then heat-treated, to form thefirst layers 65 a, as illustrated in FIG. 5D. The first layers 65 a eachhave a first surface 65 a ₁ that is electrically connected to thecorresponding first electrode 61, and a second surface 65 a ₂ having aprotrusion.

As illustrated in FIG. 5E, an acidic or basic solution is applied to theholes 71, to expand space z between the protrusions of the first layers65 a and the walls of the holes 71. The acidic or basic solution can bea solution that does not react with the first layer 65 a and canselectively dissolve the walls of the holes 71. For example, an aqueoussodium hydroxide solution can be used.

As illustrated in FIG. 5F, second layers 65 b that have a first surface65 b ₁ may be formed. The first surfaces 65 b ₁ have a portion 65 b ₃that does not contact the second surface 65 a ₂ of the first layer 65 a.

As illustrated in FIG. 5F, since the second layer 65 b is formed afterthe space z is expanded, the width of the second layer 65 b can berelatively increased, as compared to the width of the first layer 65 a.Accordingly, current-voltage characteristics of the nano/micro-sizeddiode can be improved.

As illustrated in FIG. 5G, control layers 67 are formed on the secondlayers 65 b. Then, as illustrated in FIG. 5H, second electrodes 63 areformed on the control layers 67. Lastly, the porous template 70 isremoved, as described above, to obtain a plurality of nano/micro-sizeddiodes.

Since the control layers 67 and the second electrodes 63 are formedafter the walls of the holes 71 are partially dissolved, as illustratedin FIG. 5E, the widths of the control layers 67 and the secondelectrodes 63 are the same as the widths of the second layers 65 b. As aresult, the nano/micro-sized diode 30 can be mass-produced at a highyield.

Hereinafter, aspects of the present invention will be described morespecifically with reference to the following examples. The followingexamples are for illustrative purposes and are not intended to limit thescope of the invention.

Example 1 Preparation of First Electrode and First Layer

A porous anodic aluminum oxide (AAO) template (Whatman InternationalLtd.), having a diameter of 13 mm and including a plurality of holes,was prepared. The diameter of each hole was 300 nm. An Ag thin film,having a thickness of from 200 to 300 nm, was deposited on the porousAAO template, by thermal deposition. The Ag thin film was used as aworking electrode. A platinum wire mesh was used as a counter electrode,and Ag/AgCl was used as a reference electrode.

Subsequently, an electropolymerization device (AutoLab, PGSTAT100)equipped with a potentiostat was filled with a Technic ACR silver RTUsolution (Technic, Inc.), and the porous AAO template was immersed intothe solution. A current was applied to the immersed template at a rateof 0.5 C/cm², for 5 minutes, at a constant potential of −0.9 V vsAg/AgCl, to deposit Ag in each hole of the porous AAO template. Thus, agap, between the porous AAO template and the thermally deposited Ag thinfilm, was sealed.

The prepared porous AAO template was immersed in an Orotemp 24 RTUsolution (Technic, Inc.), and then Au was electroplated thereto, at −0.9V vs Ag/AgCl, to form an Au first electrode in each hole of the porousAAO template. The thickness of the Au first electrode was set to 3000nm, by monitoring charges passing through the Au layer.

Subsequently, the electropolymerization device was filled with asolution of 0.5 M pyrrole, 0.2 M tetraethylammonium tetrafluoroborate,and an acetonitril at a positive potential. A voltage was maintained at1.0 V vs Ag/AgCl, for 10 seconds, using the potentiostat, to form alayer comprising pyrrole, tetraethylammonium tetrafluoroborate,acetonitrile, and polypyrrole, on the Au first electrodes. The thicknessof the layers was set to 700 nm.

Next, the porous MO template was dried at 80° C., for one hour, toremove the pyrrole and the solvent from the layer formed on the Au firstelectrode. As a result, first layers that were formed of polypyrrole andhad protrusions, were formed on the Au first electrodes (refer to FIG.5E). Part of the porous AAO template, in which the Au first electrodesand the first layers were formed, was sampled, and then the Ag thin filmused as the working electrode and the porous AAO template wererespectively dissolved in concentrated nitric acid and a 3M sodiumhydroxide solution. The resulting structures in the holes wererepeatedly washed, using distilled water, until the solution reached apH of 7. The resultant was observed by field emission scanning electronmicroscopy (FESEM), and the results are illustrated in FIG. 6A.

Dissolution of the Walls of the Holes

After the polypyrrole first layer was formed as described above, thewalls of the holes were dissolved using an aqueous 1M sodium hydroxidesolution, to expand a space between the protrusions of the polypyrrolefirst layers and the walls of the holes (refer to FIG. 5E).

Preparation of Second Layer

Next, the electropolymerization device was filled with an aqueoussolution, including 0.3 M cadmium sulfide, 0.7 mM selenium dioxide, and0.25 M sulfuric acid. Then, cyclic voltammetry was performed, at a scanrate of 750 mV/s and at a voltage in the range of −0.36 to −0.8 V, for4000 cycles, to reduce cadmium selenide onto the first polypyrrole layerof each hole of the porous AAO template. As a result, a second layer,which was formed of cadmium selenide and had a recess corresponding tothe protrusion of the polypyrrole first layer, was formed. The thicknessof the second layer was adjusted to make the total thickness of thediode layer comprising the first layer and second layer to be 1500 nm(refer to FIG. 5F). A part of the porous AAO template, in which the Aufirst electrode, the polypyrrole first layer, and the CdSe second layerwere formed, was sampled, and then the Ag thin film and the porous AAOtemplate were respectively dissolved in a concentrated nitric acid and a3M sodium hydroxide solution. The resulting structure was repeatedlywashed using distilled water, until the solution reached a pH of 7. Theresultant was observed by field emission scanning electron microscopy(FESEM), and the results are illustrated in FIG. 6B.

Preparation of Control Layer

An Ag control layer was formed on the CdSe second layer. The Ag controllayer was formed, by immersing the porous AAO template in a Technic ACRsilver RTU solution (Technic, Inc.). A current was applied at a rate of0.5 C/cm², for 1 minute, at a constant potential of −0.9 V vs Ag/AgCl,using the potentiostat. The thickness of the Ag control layer was 500nm. A part of the porous AAO template, in which the Au first electrode,the polypyrrole first layer, the CdSe second layer, and the Ag controllayer were formed, was sampled. Then the Ag thin film and the porous AAOtemplate were respectively dissolved in a concentrated nitric acid and a3M sodium hydroxide solution. The resulting structure was repeatedlywashed using distilled water, until the solution reached a pH of 7. Theresultant was observed by field emission scanning electron microscopy(FESEM), and the results are illustrated in FIG. 6C.

Preparation of Second Electrode

The porous AAO template was immersed in an Orotemp 24 RTU solution(Technic, Inc.); and then Au was plated at −0.9 V vs Ag/AgCl, to form anAu second electrode on the Ag control layer (refer to FIG. 5H). Thethickness of the Au second electrode was 2000 nm.

Removal of Porous Template and Yield of Diode

The Ag thin film used as the working electrode and the porous AAOtemplate were respectively dissolved in a concentrated nitric acid and a3M sodium hydroxide solution, to obtain a plurality of structures fromthe holes. Then, the resulting structures were repeatedly washed indistilled water, until the solution reached a pH of 7, to complete amanufacture of nanorod diodes (refer to FIG. 3B for a cross-section ofthe diode). The diodes were observed by FESEM, and the results areillustrated in FIG. 6D. In addition, the diodes were observed by FESEMat different magnifications, and the resulting FESEM images are shown inFIGS. 7A and 7B. The yield (number of undamaged diodes/total number ofdiodes×100) of the diodes was calculated from FIG. 7B, and as a result,the yield of the diodes was confirmed to be about 70%.

In addition, a scanning electron microscope (SEM) image of the diode atanother magnification is illustrated in FIG. 8A. Energy dispersivespectroscopy (EDS) spectra of the diode were use for analyzing thecomposition in sites corresponding to Spectrum 1, Spectrum 2, andSpectrum 3 in FIG. 8A, which are respectively illustrated in FIGS. 8B,8C and 8D. The composition analysis results of Spectrum 1, Spectrum 2,and Spectrum 3 are respectively shown in Tables 1, 2 and 3 below:

TABLE 1 element wt % atomic % Au 100.00 100.00 Total 100.00 100.00

TABLE 2 element wt % atomic % Se 6.17 9.38 Ag 62.47 69.48 Cd 4.44 4.74Au 26.91 16.39 Total 100.00 100.00

TABLE 3 element wt % atomic % Ag 59.97 73.23 Au 40.03 26.77 Total 100.00100.00

Example 2

A diode (refer to FIG. 2B for a cross-section of the diode) was preparedin the same manner as in Example 1, except that after the polypyrrolefirst layer was formed, the CdSe second layer was formed withoutdissolving the walls of the holes of the porous AAO template.

Evaluation Example 1

Current-voltage characteristic curves, in the case of irradiating whitelight and no light irradiation on the diode prepared in Example 1, areillustrated in FIG. 9. In addition, current-voltage characteristiccurves, in the case of irradiating white light and no light irradiationon the diode prepared in Example 2, are illustrated in FIG. 10 (In FIGS.9 and 10, first light refers to 100 W white light, second light refersto 200 W white light, and third light refers to 300 W white light).

From FIGS. 9 and 10, it can be seen that the nanorod diodes of Examples1 and 2 exhibit diode-type current and voltage characteristics. Inparticular, from FIG. 9, it was confirmed that the nanorod diode ofExample 1 did not have a breakdown voltage, even at −10V. In addition,from FIG. 10, it can be seen that light intensity of the nanorod diodeof Example 2 depends on the current intensity.

Although a few exemplary embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments, without departing fromthe principles and spirit of the invention, the scope of which isdefined in the claims and their equivalents.

1. A nano/micro-sized diode comprising: a first electrode; a secondelectrode; and a diode layer disposed between the first electrode andthe second electrode, the diode layer comprising, a first layer disposedon the first electrode, having a first surface that is electricallyconnected to the first electrode, and an opposing second surface havinga protrusion, and a second layer disposed between the first layer andthe second electrode, having a first surface having a recesscorresponding to the protrusion, and an opposing second surface that iselectrically connected to the second electrode, wherein the firstsurface of the second layer entirely covers the second surface of thefirst layer.
 2. A method of preparing a nano/micro-sized diode,comprising: forming first electrodes in holes of a porous template;applying material layers onto the first electrodes; heat treating thematerial layers, to form first layers that each has a first surface thatis electrically connected to the first electrode, and an opposing secondsurface that has a protrusion; forming second layers on the firstlayers, the second layers each having a first surface having a recessthat corresponds to a respective one of the protrusions; forming secondelectrodes on the second layers; and removing the porous template, toobtain nano/micro-sized diodes.
 3. The method of claim 2, furthercomprising forming a control layer on the second layer.
 4. The method ofclaim 2, wherein the heat treating of the material layers is performedat a temperature of from 25° C. to 150° C.
 5. The method of claim 2,wherein the heat treating of the material layers is performed for from 1to 24 hours.
 6. The method of claim 2, further comprising expandingspaces between the protrusions and the walls of the holes.
 7. The methodof claim 2, wherein the porous template is selected from the groupconsisting of: an anodic aluminum oxide membrane; a polycarbonatetemplate; an anodic titania membrane; and a porous membrane comprisingpolypropylene, nylon, polyester, or block copolymers thereof.
 8. Themethod of claim 2, wherein the removing of the porous template compriseswet etching, dry etching, or pyrolysis.